I suggest to keep SMTP_v3 unziped in a regular folder. It makes it a whole lot easier to add/modify the trucks.
Here are some good resources for making/editing the trucks/tracks
RULE # UNO for editing Give credit and ask first
(I know your a veteran and i'm still a 'noob' just sayin' because so many people miss this)
Paint Dot Net Free , close to photoshop $0
Sketchup Free, but there is a premium, however ts not needed
These are simple and great to start out with. There are tutorials on how to convert Sketchup made tracks to the game.